Although sensitive tests for assessing abnormalities in airway resistance exist, these tests are largely impractical for screening a large number of subjects. One such test, the forced airflow perturbation technique for measuring frequency-dependent resistance, is suited for mass testing except for the requirement of time consuming data reduction. The research proposed here is directed at developing an automated system for on-line measurement of resistance by this approach. A novel method of "resonance cancellation" using a phase sensitive automatic control loop is proposed. The completed system will be used in the assessment of effects of oxidative air pollutants on human subjects undergoing controlled exposure. Patients with chronic obstructive lung disease will also be tested.